Colorectal cancer is the development of cancer in portions of the large intestine, such as the colon or rectum. Detection of blood in feces is used as a screening tool for colorectal cancer. However, the blood is often occult blood, i.e., blood that is not visible. The stool guaiac test is one of several methods that detect the presence of blood in feces, even in cases in which the blood is not visible. A fecal sample is placed on a specially prepared type of paper, called guaiac paper, and hydrogen peroxide is applied. In the presence of blood, a blue color appears on the paper. A patient who is suspected of suffering from colorectal cancer will typically be assessed using a colonoscopy, sigmoidoscopy, and/or external imaging techniques, such as CT, PET, and/or MRI.
Bladder cancer is a condition in which cancerous cells multiply within the epithelial lining of the urinary bladder. Detection of blood in urine can be useful in screening for bladder cancer. Techniques for detecting blood include placing a test strip that contains certain chemicals into sample of the urine and detecting a color change of the test strip.